Oil and gas accumulations are found at depth in different geological basins worldwide. Exploration and production of such accumulations rely on the construction of a well according to a well plan.
Various well types exist and are defined according to usage such as wildcats or those used in exploration; delineation; and production and injection. Variations in well profile exist also according to vertical, slant, directional and horizontal trajectories. Each differs according to the oil company's objectives and the challenges that a given basin presents from the surface of the earth or the ocean to the hydrocarbon reservoir at a given underground depth.
Engineering challenges are related to the location of the well-site such as onshore or offshore, seawater depths, formation pressures and temperature gradients, formation stresses and movements and reservoir types such as carbonate or sandstone. To overcome these challenges, a highly detailed well plan is developed which contains the well objective, coordinates, legal, geological, technical, well engineering and drilling data and calculations.
The data is used to plot a well profile using precise bearings which is designed in consecutive telescopic sections-surface, intermediate and reservoir. To deliver the well objective and maintain the integrity of well over its lifecycle, a given wellbore with multiple sections and diameters is drilled from surface. Although there are many variants, a simple vertical well design could include a surface or top-hole diameter of 17½″ (445 mm), intermediate sections of 13⅝″ (360 mm) and 9⅝″ (245 mm) narrowing down to the bottom-hole diameter of 8½″ (216 mm) in the reservoir section.
Each consecutive section is ‘cased’ with the specified diameter and a number of metal tubes placed into the wellbore according to the length of the section. Each must be connected to each other after which they are cemented into the appropriately sized hole with a given tolerance. In this way, a well is constructed in staged sections, each section dependent on the completion of the previous section until the well is isolated from the formation along the entire distance from surface to the reservoir.
Scarcity of oil and gas is driving oil and gas companies to explore and develop reserves in more challenging basins such as those in water-depths exceeding 6,000 ft (1830 m) or below massive salt sections. These wells have highly complex directional trajectories with casing designs including 6 or more well sections. Known in the art as ‘designer’ or ‘close tolerance casing’ wells, these wells have narrow casing diameters with tight tolerances and have created a need to enlarge the wellbore to avoid very narrow diameter reservoir sections and lower production rates.
Therefore, the bottom-hole assemblies that are needed to drill these wells routinely include devices to underream the well-bore below a given casing diameter or other restriction. In this way, underreamed hole size has become an integral part of well construction and there is now an increased dependence on underreaming to meet planned wellbore diameters.
To those skilled in the art, it is known that underreaming activities generate uncertainty as to drilling dynamics and enlarged wellbore diameter. This is because the prior art does not provide for drilling dynamics data acting on the underreamer. If any drilling data is provided, it is in the pilot hole or where drilling has already taken place. Consequently, the lack of underreaming dynamics and vibration data results in a limited understanding of the actual loads and forces acting on the underreamer in real-time.
The present invention is differentiated in this aspect as it provides real-time drilling dynamics data of the underreaming operation which would enable a driller to identify a vibration mode and change drilling conditions to eliminate the vibration. This would reduce harmful vibrations which are a major cause of downhole tool failures as well as the lower rates of penetration related to underreaming in medium to hard formations or interbedded formations.
The data maybe transmitted in real-time by means of data transfer (mud pulse telemetry, fibre-optic or other) or maybe stored in memory and downloaded at a later time. The present invention would make underreamed drilling data available and optimize underreaming operations thereby reducing failures and improving drilling efficiency.
The present invention provides for a novel approach that measures underreaming drilling dynamics and pilot hole drilling dynamics considering the two as separate but inter-related with each subject to different levels of vibration, weight, torque, bending moments, accelerations etc. Typically, an underreaming run has no means of investigating drilling dynamics at the underreamer and therefore corrective actions are limited to reducing the rate of penetration, reducing the rotary speed, reducing the weight on bit.
Drilling vibration occurs in two modes which are classed as resonant i.e. torsional, axial, lateral and non-resonant vibration, i.e. eccentric, sudden modes releasing stored torque known as backward rotation and whirl. Each mode maybe defined separately but in reality all modes are experienced during drilling, the severity of each level depends on the drilling BHA design, drilling dynamics and geological formations. Axial vibrations are often manifested as a stick-slip condition where weight applied at surface causes compression of the drill-string. Due to the geometry of the wellbore and bending moments, the weight is concentrated at the widest diameters of the drill-string which is the underreamer, stabilizers, rotary steerables and the drill-bit.
By detailing the exact nature of vibration at the underreamer, the present invention would be able to alert the driller when vibration reaches a certain level, therefore highlighting a change in drilling parameters or geological conditions that have caused such a change. Previously, this would not be possible and drilling delays could occur.
Previously, to overcome the lack of underreaming drilling data the industry has relied on models of the bit and underreamer wear in varying formations to investigate the effects that formation loading has on both. However, the models are based on predicted values rather than actual measurements. In this way, the present invention would not only optimise drilling during underreaming operations but also provide for accurate input data for modelling drilling dynamics.
The present invention would characterize underreamer dynamics during drilling and would highlight wear and avoid premature failures due to tool design weak spots or usage related to surface or downhole parameters as well as to optimise the rate of penetration and hole cleaning.
In the aspects of a sensing underreamer tool capable of measuring drilling dynamics and eliminating downtime and premature failures, the present invention is differentiated from the prior art underreamers. These are unsatisfactory as they there are no actual underreaming drilling dynamics measurements whether direct or inferred.
It is unsatisfactory to depend on indirect indicators such as whether cutter blocks are open or closed or whether fluid pathways are open and a pressure spike is seen at the rig floor to indicate activation. Such indicators do not provide actual measurements of the underreamed well-bore underreamed nor do they provide verification of underreaming performance; they simply give information on the mechanical or hydraulic status of an aspect of the tool which may or may not lead to the desired well diameter.
To those skilled in the art, it is known that the industry relies on even more rudimentary and time-consuming indicators of verification such as an increase in drilling torque as cutters interact with the formation or even pulling up the drill-string and underreamer to the previous hole size in order to see whether the top-drive stalls as the bottom-hole assembly gets stuck due to the expanded tool. Or by drilling a pilot hole section with the underreamer deactivated and pulling back into the pilot hole.
In the specific aspect of drilling dynamics and underreaming behaviour, the ability to acquire accurate data on vibration, stick-slip, loads and other accelerations by means of the present invention optimally differentiates it from prior art. Prior art underreamers do not provide for in-situ data measurements on drilling dynamics or loads and therefore their performance, design or configuration cannot be optimized from one job to another.
A major drawback of prior art underreamers is their cutting performance in terms of maintaining similar rates of penetration to the bit. To one skilled in the art, it is known that the bit has optimized designs due to the well characterized drilling dynamics which is well understood by means of measurement-while-drilling data acquired in the pilot-hole. Further cutter element placements and nozzle locations are optimized in the drill-bit and not necessarily so in the underreamer and this often leads to the bit outperforming the underreamer.
Consequently, this leads to either a separate underreaming run after drilling the section, or leads to the underreaming-while-drilling itself taking several attempts before a complete section is underreamed satisfactorily. In terms of drilling dynamics, the optimised drill-bit drills at a faster rate of penetration than the underreamer which has trouble maintaining similar rates of penetration. Due to the distances between the bit and underreamer which maybe 120′ or more, the bit may have exited a hard formation or layer while the underreamer may just be entering the earlier hard formation or hard layer as it is may not be connected directly to the bit. The prior art underreamers do not provide for measurements of the drilling dynamics concentrated at the underreamer.
Therefore, the prior art does not lend itself to a reliable or certain means of measuring underreaming drilling dynamics in real-time or memory mode.
Further the prior art perpetuates drilling inefficiencies due to the uncertainty of actual vibrations at the underreamer.
Further the prior art does not provide for accurate input into underreaming and drilling models.
Further the prior art does not allow for the timely identification of vibration modes or their correction in real-time.